1. Technical Field
This invention pertains to thickness gauges for use in measuring the thickness of a moving sheet of material such as a paper web. Such gauges may be broadly classified as contacting or non-contacting. This invention relates to the former class. More particularly, the invention relates to gauges of the former class which employ surface followers (sheet-contacting components of contacting thickness gauges) designed to mimimize damage to the sheet and to preserve the surface characteristics of electrically conductive proximeter targets employed in such gauges.
2. Discussion of Related Art
Thickness gauges, also known as caliper gauges, are well known in the field of sheet-process measurement. Typical contacting thickness gauges are described in U.S. Pat. Nos. 3,525,929 Mounce, 3,828,248 Wennerberg, 4,107,847 Typpo et al., and 4,134,211 Typpo et al.
These gauges typically sense the thickness of the sheet by incorporating some design in which the distance between a proximeter probe (typically, an induction coil forming a part of a proximeter circuit) and a proximeter target (typically comprising a piece of ferrite or other electrically-conductive material, or a coil) varies with the thickness of the sheet. Changes in thickness are determined from changes in electrical signals produced in the associated circuit, which result from the variations in distance. The variation in distance between the probe and the target is effected by the movement of one or more surface followers which maintain substantially continuous contact with the sheet.
Various means have been adopted in attempts to minimize damage resulting from contact between the surface follower and the sheet. The surface followers are typically rounded or beveled, and may be secured to bellows, diaphragms, and/or mechanical linkage so that the follower can move in response to impact with a passing protruberance in the sheet.
Where the surface follower serves as the proximeter target (as in U.S. Pat. No. 3,828,248, for example), the continuous contact between its sheet-contacting surface and the sheet gradually wears away the contacting surface. This can be expected to change the surface characteristics of the surface follower in what, in all likelihood, will be a non-uniform manner. The change in surface characteristics can in turn be expected to produce gradual changes in the paths of eddy currents flowing in the target, thus producing a source of error in the thickness measurement.